1. Field of the Invention
The invention relates to a magnetic ("MR") resonance system for interventional procedures, including an MR device which is arranged to acquire images of a region of an object, the MR device including:
a) a magnet for generating a steady magnetic field, PA1 b) means for generating temporary magnetic gradient fields, comprising a read gradient and other temporary gradient fields orthogonal to the read gradient, PA1 c) means for generating RF-pulses, PA1 d) means for measuring MR signals, PA1 e) processing means for processing the MR signals measured so as to determine an image of the object, PA1 f) an invasive device which comprises means for generating an auxiliary magnetic field and PA1 g) a control unit for generating control signals for the means for generating the temporary gradient fields, the means for generating the RF-pulses and the means for generating the auxiliary magnetic field, the control unit being arranged such that the read gradient and one of the other temporary gradient fields are superposed on a steady magnetic field so that during measurement of the MR signal the k-space is scanned along a plurality of lines, and the auxiliary magnetic field is applied, the processing means further being arranged to determine a position of the invasive device within the object from changes in the measured MR signals as induced by the applied auxiliary magnetic field.
2. Description of the Related Art
All MR system of this kind is known from U.S. Pat. No. 4,572,198. It is to be noted that in the context of the present Patent Application an invasive device is to be understood to mean any instrument whose positioning is of importance, for example an instrument such as a guide wire or a biopsy needle. In the context of the present Patent Application k-space is to be understood as a spatial frequency domain in which an MR signal is measured along a trajectory and the measured values yield the inverse fourier transformed values of the image of the object. The trajectory in k-space is determined by the time integral of the applied temporary gradient fields over a time interval from the excitation RF-pulse to the actual measurement point in time of the MR signal.
In the known MR system the invasive device is positioned, in cooperation with the MR device, so as to subject the object to an interventional procedure for which the invasive device has been designed. This is, for example, angioplasty of a patient. Furthermore, in the known MR system the image processing means determines the position of a part of the invasive device in a slice of the object on the basis of two consecutive MR images of the patient. The auxiliary magnetic field is then switched off during the generating of the MR signals for the reconstruction of a first image, whereas it is switched on during the generating of the MR signals for the reconstruction of a next MR image. The auxiliary magnetic field generated by a coil provided in the part of the invasive device disturbs the magnetic fields generated by the MR device, so that a difference arises between the two MR images. The processing unit determines the position of the part of the invasive device on the basis of the changes in the two MR images. In a further step the position of the part of the invasive device is superposed, via a cursor, on the MR image of the body and displayed on a monitor.
It is a drawback of the known system that the accuracy of the position determined is affected by motion artefacts in the image of the invasive device or the object.